We have a minivan and a Prius. We can fit all three kids in the back of the Prius, but it's a pain to buckle the older ones into their boosters, and it can be a pain to keep them happy and not fighting in such close quarters. We find ourselves taking the minivan most of the time. In the van, there is less fighting, built-in entertainment, and the older 2 kids can buckle themselves independently. The Prius is used mostly as a commuter car or if just one parent and one child are going somewhere while the other parent/kids stay home.
ETA: just wanted to mention we (especially DH) are generally quite energy conscious and he is obsessive about getting good gas mileage and stuff. He even rides his bicycle to work (15 miles each way) partially because he gets a kick out of saving gas (vs driving the Prius!)
So for us to choose driving the van while the Prius sits at home, that goes to show how much more convenient it usually is to load kids into and out of a minivan vs a small, fuel efficient sedan.